1. Field of the Invention
The present invention relates to downhole tools for sensing the stresses caused by torque and compression acting on the drill string, and for minimizing steady state errors due to pressure and temperature differences.
2. Description of the Prior Art
Weight-on-bit is generally recognized as being an important parameter in controlling the drilling of a well. Properly controlled weight-on-bit is necessary to optimize the rate that the bit penetrates the formation, as well a the bit wear.
Torque also is an important measure useful in estimating the wear of the bit, particularly when considered together with measurements of weight-on-bit. Excessive torque is indicative of serious bit damage such as bearing failure and locked cones.
In the past, weight-on-bit and torque measurements have been made at the surface. However, a surface measurement is not always reliable due to drag of the drill string on the borehole wall, and other factors.
Recent developments in borehole telemetry systems have made it possible to make the measurements downhole, but for the most part, the downhole sensors that have been utilized are subject to significant inaccuracies due to the effects of well pressures and temperature gradients that are present during the drilling process. These systems, regardless of the design of the sensing equipment cannot distinguish between strain due to weight and axial strain due to pressure differential "pump apart" force. This force may be defined as the force on the end area of a cylindrical pressure vessel such as an oil well drill pipe string which urges said vessel to elongate under internal pressure.
The problem that leads to the employment of a mechanical strain amplifier is that of obtaining a signal of satisfactory magnitude. Sensitive strain elements are subject to damage at high loads.
The first design adapted to this problem is described in U.S. Pat. No. 3,686,942. In that design the strain element is limber enough to give good signal response but the travel of its motion is constrained with stops to prevent inelastic deformation for loads well beyond the range of interesting measurements.
Another approach to this problem is shown in U.S. Pat. No. 3,968,473. This patent describes a tool having an inner mandrel with a thin section on which strain gages are glued and an outer stablizing sleeve. While there is no mechanical amplification in this design, the patent describes a mathematical sizing of the strain element so as to obtain matched sensitivity in the weight-on-bit and torque-on-bit modes at the maximum needed strength.
U.S. Pat. No. 3,827,294 shows a mechanical strain amplifier in a downhole tool which is geometrically dissimilar to the one disclosed in the present specification. Mechanical strain amplifiers are also shown in U.S. Pat. Nos. 3,876,972 and 4,608,861.
U.S. Pat. Nos. 4,359,898 and 3,968,473 illustrate designs utilizing pressure compensating devices, which, again, are dissimilar to the device disclosed in the present specification.
The current devices described above are deficient in at least one of the following features: automatic pressure compensation to correct for axial stress which is caused by "pump apart" tension; a means to prevent circumferential stress due to bore pressure from distorting the axial force bridge reading; and a means to avoid the effects of tool distortion due to temperature gradients.